1. Field of the Invention
The invention relates to a valve mechanism, in particular for internal combustion engines of motor vehicles, having at least one driven cam element and having a valve actuator, which can be displaced or pivoted by the cam element, the cam element being arranged rotatably in a flexible surround element which is connected to the valve actuator in such a manner that it can move in a plane which is perpendicular to the axis of rotation of the cam element.
Valve mechanisms for controlling the valves of internal combustion engines, in particular for motor vehicles, usually have a device (spring, hydraulic element, etc.) which is used to load the valve toward its closed position. In this position, a valve actuator (valve lifter, drag lever, rocker lever or the like) is pressed against a continuous valve control surface, which in part runs eccentrically with respect to the shaft axis. When the valve is closing, it should be ensured that the valve disk does not strike the valve seat too quickly, since otherwise it will rebound. This requires relatively complex matching between the shape of the cam, the masses which are to be moved, the forces which are generated, the materials properties, etc.
Therefore, there is no lack of proposals concerning forced guidance of the valve actuator on the cam element; various embodiments have been developed, which are each based on two eccentric valve control surfaces instead of the restoring spring. Specific designs are to be found, for example, in British patent specifications GB 19,193(1913) and GB 434,247, wherein the cam element, on at least one end face, has a groove, the two side walls of which form the valve control surfaces. A roller or the like which is arranged at the end of the valve actuator engages in the groove from the side. A cam element which has a web which can be gripped around is known, for example, from European publication EP 429 277 A.
A further proposal for a desmodromic valve mechanism, wherein a space-saving, lightweight and inexpensive design is achieved, is to be found in published German patent application DE 37 00 715 A which describes the generic type referred to in the introduction. In this design, a surround element is provided, which surrounds the circumference of the cam element without significant play, so that it always matches the shape of the cam, yet the cam element, on account of the nature of the surround element, can rotate inside the latter. Since the surround element connected to the valve actuator cannot rotate with the cam element, the movement of the cam region about the axis of rotation of the cam element is converted into a lifting or reciprocating movement of the valve actuator which is mounted displaceably or pivotably in the cylinder head. The valve actuator does not execute a movement as long as the connecting region of the surround element together with the valve actuator rests against the base circle region of the rotating cam element, is then moved away from the axis of rotation of the cam element in the radial direction and finally is returned again, while the cam region of the cam element moves past the connecting region of the surround element and the valve actuator. The moveable connection of the surround element to the valve actuator allows the pivoting or tilting movement of the surround element in the cam region, so that the required freedom of movement of the valve actuator in its sliding or pivot bearing is preserved. In the first exemplary embodiment, the surround element is formed by two flexible rings, between which needle-shaped rolling bodies are provided in order to reduce the friction. A second embodiment provides a plastic strip having an inner ceramic slip layer.
Particularly when the valve mechanism is used in internal combustion engines, a surround element is subject to high loads, and it is necessary to rule out temperature- or fatigue-related plastic lengthening of the surround element. An irreversible increase in the size of the gap between the circumference of the cam element and the surround element affects in particular the valve-closing position.
Furthermore, the term variable valve control has revealed a wide range of different structures which can be used to change the opening and closing time and the lift of the valve, in order to improve the performance, the exhaust emissions, the torque, etc. of an internal combustion engine. Compared to the non-adjustable valve control with fixed values, the filling of a cylinder is improved if the valve is opened later and closed earlier at low rotational speeds and is opened earlier and closed later at higher rotational speeds. It is therefore possible, by means of a speed-dependent adjustment of the valve control, to optimize the exhaust emissions, the torque, the engine performance, etc. All the variable valve control arrangements which have been revealed to date change the position of the actuating surface of the valve actuator relative to the eccentric valve control surface through rotation, linear displacement or enlargement of the cam element. These adjustment mechanisms are relatively complex and, in some cases, also require considerable adjustment forces, since they have to operate counter to the restoring elements of the valves.
It is accordingly an object of the invention to provide a valve lifting mechanism, particularly for an internal-combustion engine, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for a variable forced valve control.
With the foregoing and other objects in view there is provided, in accordance with the invention, a valve mechanism, comprising:
at least one driven cam element and a valve actuator driven by said cam element;
a flexible surround element, said cam element being rotatably disposed in said flexible surround element about an axis of rotation and said flexible surround element being movably connected to said valve actuator for movement in a plane perpendicular to said axis of rotation of said cam element, and wherein said surround element is configured to be reversibly lengthened for adjusting a valve lift of said valve actuator.
In other words, the objects of the invention are achieved, in a valve mechanism of the type described in the introduction, in that the surround element is designed so that it can be lengthened reversibly.
The cam element rotating in the surround element generates tensile forces, which rise as a function of rotational speed, at the connecting point to the valve actuator, so that the surround element, which bears against the circumference of the cam element virtually without play at idling speed, is lifted increasingly further away from the circumference as the rotational speed rises, thus adopting positions which correspond to cam elements with greater circumferential lengths. Since, in this way, the distance between the axis of rotation of the cam element and the connecting point between the surround element and the valve actuator increases, an additional valve lift which is dependent on rotational speed is produced.
In a first embodiment, the reversible lengthening of the surround element is achieved by elastic stretchability of at least a partial region of the surround element, so that the play which is formed between the cam circumference and the surround element is reduced further as the rotational speed falls. Moreover, it allows advantageous, slight prestressing of the surround element in the at-rest state, in order to ensure that the valve-closing position is reached outside the cam region despite any temperature-related changes in length.
The surround element may consist of an elastically stretchable material or may be composed of two materials with different properties, at least one of which can stretch elastically. By way of example, a non-stretch strip may be closed with respect to the surround element by an elastically stretchable intermediate piece, wherein case a holder for the valve actuator may be provided either in the nonstretch region or in the elastically stretchable region. If the holder is in the elastically stretchable region, it may itself also consist of an elastically stretchable material and, if appropriate, may also form the elastic region.
For internal combustion engines of motor vehicles, the elastically stretchable material is preferably designed for an additional valve lift of 10% to 30% of the valve lift at idling speed. In order, in a preferred embodiment, to ensure an upper limit value of the elastic stretching, which can be selected for a permissible maximum rotational speed or a rotational speed above which an additional valve lift is of subordinate importance, a stretch-limiting means can be assigned to the elastically stretchable material by arranging nonstretch filaments or fibers, the length of which corresponds to the length of the elastic material which has been stretched to the limit, in or parallel to the stretchable material.
In a second embodiment, the surround element has a protuberance which is formed by an elastically resilient constriction, the tensile forces acting on the holder of the valve actuator causing the elastically resilient constriction to widen. The reduction of the constriction lengthens the surround element, which in this embodiment may itself be of nonstretch design. The holder is preferably arranged in the protuberance, with the result that the two regions of the surround element, which, at idling speed, come into contact with one another between the cam element and the holder, move away from one another as the rotational speed increases and move closer to one another as the rotational speed falls.
In a further embodiment, it is provided that the surround element has a strip comprising a textile-bound sheet material, in particular comprising a woven fabric, the two ends of which are connected to a holder for the valve actuator. When the two ends of the strip penetrate through one another or project from the cam element in contact with one another, the flexibility of the material of the surround element means that a physical axis in the connection to the valve actuator may be unnecessary, since the two ends together can be bent to both sides to the required extent. For connection to the valve actuator, it is preferable for the two ends of the strip to have plug-in openings for a connecting element. The plug-in openings may be formed by winding round andxe2x80x94depending on the material used for the stripxe2x80x94sewing, adhesively bonding or welding the wrapped-around end, or the like. A particularly advantageous embodiment provides for the strip to comprise a continuous loop which is guided backward and forward about the cam element and the reversal points of which form the plug-in openings. The connecting element may also be of elastically resilient design and consist, for example, of spring steel.
If the surround element consists of two different materials, the textile-bound sheet material may have a nonstretch region, wherein it contains filaments of Kevlar(copyright), glass, carbon or aramid fibers, or the like, substantially constant-length fibers, extending in the circumferential direction of the cam element.
A surround element which forms a continuous loop may consist in particular of a sheet material which is produced using a textile circular working technique (circular weaving, circular knitting, etc.) and is provided with a holder for the valve actuator.
The elastic stretching of the loop may be selected to be linear, progressive or degressive, for example by incorporating filaments with different stretching properties, which become active simultaneously or in succession.
Further possible options provide an elastically stretchable cord or an elastically stretchable ring made from plastic, which is preferably provided with a recess for a bearing pin of the valve actuator. The plastic ring may be fiber-reinforced and/or provided with a slip-reducing metal coating. As an alternative, it is also possible to use a flat belt, in particular a ribbed belt, between the transverse ribs of which there is space for the bearing pin of the valve actuator, which is fixed by an adhesively bonded cover strip or the like. The ribbed belt may also be fitted in such a way that the ribs are internal, which eliminates the need for additional fixing of the bearing pin.
Materials which are particularly suitable for a surround element which has at least elastically stretchable partial regions have a modulus of elasticity of between 1 and 4000 N/mm2. Rubber-like materials have low moduli of elasticity and are preferably provided with a stretch-limiting means.
Materials such as plastics which have higher moduli of elasticity, in particular between 600 and 2000 N/mm2, preferably between 800 and 1200 N/mm2, do not generally require a stretch-limiting means, although it is, of course, possible to provide such means.
A simple possible option for the stretch-limiting means consists in assigning nonstretch filaments of Kevlar(copyright), glass, aramid fibers or the like, which extend in the circumferential direction and are, for example, woven into a strip, to the surround element or the elastically stretchable region of the surround element. Specifically in this design, it would also be possible to use an elastomeric plastic, which is vulcanized to the strip, for the ring or flat belt.
For internal combustion engines wherein the cylinders have two intake or discharge valves which operate in parallel, the valve pairs may have different stretching levels, for example one stretch-limited valve under partial load and the other valve without stretch-limiting means or with stretch-limiting means at full load.
If the surround element consists of a material with a low-friction surface or a surface which has been provided with a low-friction coating, it may be that lubrication of the sliding surfaces, i.e. of the circumferential surface of the cam element and of the inner surface of the surround element which bears against it, will not be required. If lubrication is required or desirable, it is preferable for the cam element to have at least one oil bore which runs radially with respect to the axis of rotation and opens out on the circumference of the cam element, inside the flexible surround element. Since the surround element does not rotate, external supply of oil through the surround element via a flexible line is also conceivable.
Instead of a film of lubricating oil, it is also possible to build up an air cushion surrounding the cam element by means of compressed air. This may be advantageous in particular in the case of a surround element made from plastic or woven plastic fabric.
In the valve mechanism according to the invention, the masses which have to be accelerated are reduced by the elimination of the valve spring and spring disk and by a significantly lighter design of the valve lifter or rocker lever. The use of light metals, ceramics or plastics for the valve and/or the valve actuator allows the masses which have to be accelerated and decelerated to be reduced by from 50% to 80% of the value for a valve lifter with restoring spring and hydraulic play compensation. The high values result in particular in the part-load range, since the valve springs have to be designed to be able to withstand full load. Furthermore, the valve may be of shorter design, since the bulky valve spring is eliminated.
It is also possible for the cam element to be of shorter design. It also becomes possible to form plastic cam elements or camshafts which are produced completely from plastic, for example by injection molding. The use of other lightweight materials for the production of the camshafts or of the cam elements, for example aluminum, also becomes possible. On account of the reduction in mass and the lubrication, fuel savings of 5% and more are to be expected.
Particularly if valve actuators are actuated together, it is possible to provide a weak spring for acting on each closed valve.
According to another feature of the invention, the cam element has a circumference variably adjustable in correspondence with a length of the surround element.
According to yet another feature of the invention, the cam element has a cam circumference surface with a cylindrical region and an eccentric region; the cam element has a first cam part and a second cam part, the first and second cam parts are movable relative to one another; and the first cam part includes the cylindrical region of the cam circumference surface, the second cam part includes the eccentric region of the cam circumference surface.
According to a further feature of the invention, a camshaft has an inclined camshaft surface, the camshaft is movable along the longitudinal direction in a support shaft; the second cam part is guided in the first cam part and can be pushed radially outwardly; and the second cam part has an inclined surface cooperating with the inclined camshaft surface.
According to another feature of the invention, a camshaft has a spiral-shaped control surface, the camshaft being rotatable in a support shaft; the second cam part is guided in the first cam part and can be pushed radially outwardly; and the second cam part has a bridge cooperating with the spiral-shaped control surface of the camshaft.
According to yet another feature of the invention, a support shaft defines an axis of rotation; a camshaft has a crankpin and is rotatable in the support shaft; and the second cam part is disposed at the first cam part and is pivotable about an axis parallel to the axis of rotation, the second cam part has a guide surface cooperating with the crankpin.
In the previously described exemplary embodiments the lengthening of the surround element takes place exclusively by the tensile forces which increase in dependence on the rotational speed at the junction with the valve actuator, such that it lifts off from the cam element.
This lifting off which may be disadvantageous with regard to an uninterrupted lubricating film is eliminated when the circumference of the cam element is lengthened to the same extent. A lengthening of the circumference can be achieved in particular in that the cam element is constructed as having at least two parts and in that a part which carries the eccentric valve control surface can actively be adjusted with respect to a distance to the axis of rotation of the cam element for example in that the part which carries the eccentric valve control surface can be radially pushed inward or outward or can be pivoted inward or outward about an axis which is parallel to the axis of rotation of the cam element. Operating mechanisms for adjusting a part which carries the eccentric valve control surface are known from variable valve controls (U.S. Pat. Nos. 2,888,837 and 3,489,032).
In a first embodiment of the invention, the operating mechanism includes a longitudinally adjustable camshaft with an inclined surface which cooperates with a corresponding inclined surface at the adjustable part of the cam element. In a further embodiment, the control shaft can be rotated relative to the support shaft and can include a crankpin-like element which moves the adjustable part out or in. If the part is pivoted, an asymmetrical adjustment of the cam profile takes place which additionally results in different valve opening and closing characteristics. The active adjustment of the valve lift allows an increased adjustment range due to the fact that a no-stroke or xe2x80x9czero liftxe2x80x9d can be chosen as a basis, wherein the parts of the cam element fit into each other within a circular peripheral outline. The no-stroke, for example, is important if it should be possible to turn off individual cylinders in a combustion engine.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a valve mechanism, in particular for internal combustion engines, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.